Method for Oxygen Delignification of Cellulose Pulp by Mixing of Chemicals

ABSTRACT

The method is for the improved oxygen delignification of cellulose pulp with a medium consistency of 8-16%. The fraction of dissolved oxygen can be maintained at a high level throughout the process by the use of high pressure, greater than 15.0 bar, and by repeated agitative mixing while maintaining the high pressure. A fraction just over 20% of the total oxygen added is dissolved in the fluid phase such that the amount of oxygen in the fluid phase is maintained at a high level throughout the complete high-pressure section. This means that the amount of oxygen that is dissolved in the fluid phase and that penetrates the cellulose fibers can be maintained at an optimal high level throughout the process for improved delignification of the cellulose.

The present invention concerns a method for the improved oxygendelignification of cellulose pulp according to the introduction to claim1.

THE PRIOR ART

The first system with two-stage oxygen delignification was tested inMoss in Norway, and the first commercial two-stage system with remixingbetween the stages was subsequently implemented at Tomakomai mill inJapan, the results of which were reported in the Tappi Proceedings Sep.8-10, 1992, Japan-Tokyo, pp. 23-31. The principal aim of this remixingbetween the stages, between the reactions, was to finely distributeresidual chemicals in the pulp suspension and to break up any large gasbubbles to finely divided gas bubbles. Kvaerner Pulping (under its nameat the time of “Kamyr AB”) took part in both of these systems, the oneat Moss and the one at Tomakomai, as supplier of MC-mixers and MC-pumps.

One of the first patents covering two-stage oxygen delignification isdisplayed in U.S. Pat. No. 5,217,575, owned by Kvaerner Pulping AB,where the principles of using an active heating before the second stageare reported. It is there described in figures that improveddelignification is obtained through successively increased externalheating between the stages or reactions, where the patent protects amore than approximately 20 degrees higher temperature in a secondreactor. This, naturally, excludes the exothermic heating that theoxygen delignification reaction gives rise to of itself. Typically, anexothermic reaction takes place, which gives a temperature increase of5-8° C. in the region of medium consistency, given an input kappa valueof 25-30 units, which exothermic reaction is thus not included. It isspecified in U.S. Pat. No. 5,217,575 that a pressure of approximately 5bar is established in the two reactors, which creates, in practicalapplication in such systems having only one pump before the firstreactor, a pressurisation of 5-8 bar in the first reactor and apressurisation of 4-6 bar in the second reaction due to the large fallin pressure through the system that is caused by the viscously flowingpulp of medium consistency. A fall in pressure of 0.05-0.1 bar per meterof pipe is typically developed in these systems. The second mixerbetween the reactors was primarily intended to provide a remixing ofresidual chemicals and thus not necessarily with the addition of furtherchemicals, where the second stage becomes more of an extended alkaliextraction with residual chemicals from the first stage.

A later system is revealed through, for example, the patent SE505141(equivalent t0 U.S. Pat. No. 6,221,206; U.S. Pat. No. 6,319,357 and U.S.Pat. No. 6,454,900) in which at least 25 kg of alkali and 25 kg oxygenper tonne of pulp is to be initially added batchwise in a system withtwo reactors placed in series. Only one remixing takes place between thereactors, but this may take place with the addition of a small amount ofalkali. A higher pressure is established in the first reactor in thesesystems with a highest specified pressure of 10 bar and a lower pressureof a maximum of 5 bar in the second reactor. The same applicant, SundsDefibrator, now named “Metso Paper”, has demonstrated in the subsequentpatent SE 510740 (equivalent to U.S. Pat. No. 6,238,517) a secondvariant with 4-15 bar in the first reactor and 2-5 bar in the secondreactor, and the applicant has described a third and a fourth variant inSE 507871 and SE 507870 with 3-10 bar in a first reactor of upward flowand less than 2 bar in a second reactor of downward flow. It can be seenfrom this plethora of variants from the same applicant that theapplicant has not realised the significance of the maintained highpressure in a second reactor following a remixing.

The maximal pressurisation of 12 bar follows the theories that have beenadhered to by, for example, Metso Paper, and that are presented by Olmand Teder (Tappi Proceedings Seattle, 1979, pages 169-179), in which itis reported that no advantageous effects can be demonstrated at a higherpressure than 10-12 bar. It must, however, be mentioned that thesetheories were established principally after laboratory experiments inwhich autoclaves were used in which only a small amount of pulp wasbleached in a pulp specimen that was placed in the autoclave and rotatedcontinuously or, laboratory mixers have been used that have contained asmall amount of pulp specimen under continuous stirring and often undercontinuous pressurisation with externally added oxygen (something thatcreates an unlimited access to oxygen).

Theories are presented in the patents WO97/17489 (STORA) and U.S. Pat.No. 3,725,194 (SAPPI) concerning the positive effect of applying highpressure during the oxygen delignification with the aim of increasingthe amount of oxygen that, at least initially, can be dissolved in theliquid phase. A pressure of 15-20 is applied in STORA's application,while SAPPI's patent describes pressures of up to 400 psi (approximately27 bar). Also these solutions attempt to achieve an improveddelignification in which the fraction of oxygen that is dissolved in thefluid phase is high.

A specific sequence O-C/D-O-D is patented in another patent, SE 369746.The first and the second oxygen stages in this sequence are carried outvery aggressively at a pressure of 14 bar and temperatures of 119° C.and 130° C., respectively.

A two-stage oxygen delignification is described also in EP 865531, whererepeated mixing takes place between reactors. It is, however, specifiedin this case that the pressure lies within the interval from 20 psig tothe maximum pressure of 180 psig (i.e. from 1.37 to 12 bar). Thesignificance of repeated mixing at high pressure in order to optimisethe fraction of dissolved oxygen has not been recognised at all here. Amixer followed by a reactor provided with a stirrer is shown in avariant shown in FIG. 4 of the patent, which stirrer can contribute to arepeated stirring effect in the reactor.

A process is suggested also in Camilla Rööst's thesis, The Impact ofExtended Oxygen Delignification on the Process Chemistry in KraftPulping, ISSN 1652-2443, May 2004, with two oxygen reactors in which anoptimisation has been attempted of the two-stage technology according toU.S. Pat. No. 5,217,575 with heating between the stages, and in this waya powerful delignification system in which a pressure of 16 bar isestablished in the first reactor and a pressure of 6 bar has beenestablished in the second reactor has been seen. Pressures of 6, 10 and16 bar in the first reactor have been tested during the optimisation,and it has proved to be the case that the pressure, together with theaddition of alkali, is one of the most dominant parameters for gooddelignification.

Other solutions have been presented with repeated remixing during oxygendelignification. A system with three mixers in series, in which apressure of 120 psig (approximately 8.2 bar) is established, isdescribed in U.S. Pat. No. 5,460,696. The aim in this case is to reducea pH that is far too high if all alkali is added batchwise at thebeginning, and the repeated mixing takes place with the aim of mixingalkali in gradually, as the alkali is consumed.

Three mixers in sequence are shown in U.S. Pat. No. 4,384,920 and U.S.Pat. No. 4,363,697, where these mixer stations are constituted byhorizontal reactors with an internal feed screw and stirring screw. Thepulp is fed through the horizontal reactors while a gas phase isestablished at the roof of the reactors. These systems are clearly runat a moderate excess pressure.

A system is shown in U.S. Pat. No. 4,259,150 in which the pulp is leddirectly from the digester, while maintaining full digester pressure,through four mixers in series, for the addition of oxygen. In-linedrainers are used in this case to drain off fluid with precipitatedorganic material, and this has the result that the concentrationsuccessively rises. This system, however, is very expensive since largequantities of oxygen are required since the pulp suspension on exit fromthe digester contains a very large fraction of oxidisable organicmaterial in the fluid phase, and significant amounts of fibre bundlesfor which the fibre removal process, the digestion process, isincomplete, and the bundles have a high content of lignin, accompany thepulp since no straining operation precedes the oxygen treatment, whichbundles of fibres consume large quantities of oxygen. It is here claimedthat it is possible to adjust the digestion process such that thedigested pulp obtains an increased kappa value around 70, rather thanthe normal value of 35, after which delignification can take place downto a kappa value of 15. An improved selectivity can be obtained in thismanner, i.e. it is possible to reach the same kappa value but with ahigher pulp strength. This process is not one that has been driven to agreat degree, since most of the oxygen delignification stages in thebleaching line act on cellulose pulp that has an input kappa value ofthis level.

There have been executed innumerable experiments in which it has beenattempted to drive the delignification at medium consistency furtherthan what it has become clear is possible to achieve in a millenvironment. Verification of new processes often takes place inlaboratories using autoclaves or laboratory mixers, which (in contrastto the continuous processes in a mill) often takes place undercontinuous stirring of the pulp specimen and in certain cases withpressurisation by oxygen from an external source, which ensures anexcess of oxygen during the complete process, and in the presence ofsaturated oxygen in all parts of the treated pulp specimen. It is oftentherefore possible to drive the oxygen delignification further inlaboratory specimens than in a mill environment.

The mixing of the chemical, oxygen and alkali, has been considered bysome to be significant, and several solutions involve at least one ofthe establishment of a high pressure and the addition of surfactants inorder to maintain the finely distributed gas phase evenly distributedthroughout the cellulose pulp during the process. Theories have beenpresented in which a maximal contact area is to be maintained betweenthe gas phase and the fluid phase, something that can take place throughminimising the size of the finely divided gas bubbles, such that thedissolving of the oxygen in the gas phase into the fluid phase can bepromoted over a maximised transition surface. The physical process inwhich the oxygen is dissolved proceeds, however, relatively slowlycompared with the rate of consumption of oxygen during the first phasesof the reaction, and it requires that the fluid phase that locallysurrounds the oxygen bubble has a lower level of dissolved oxygen thanis theoretically possible in the process conditions that are prevalent.The oxygen is considered, however, to react primarily with the lignin inthe fibre (after the oxidisable material in the fluid phase hasreacted), and that the fluid phase that has penetrated the fibre is notimmediately surrounded by oxygen bubbles, which ensures that the oxygenthat is to react with the fibre material (and to reduce the kappa value)must first pass into solution from the gas phase to the fluid phase, andthen diffuse into the fibre in the fluid phase, and that all of this isto take place without the dissolved oxygen being consumed first byorganic material in the fluid phase. This results in the fibre having aconstant low value of dissolved oxygen with which it can react,something that is not advantageous for the process.

Aim and Purpose of the Invention

The present invention intends to improve the oxygen delignification atmedium consistency, in the region of pulp consistency 8-16%, hereafterreferred to as “MC”, in a mill environment such that delignification canbe driven as far or further than what is possible when testing inlaboratories. It is also possible with the invention to maximise in amill environment the fraction of dissolved oxygen in the fluid phase andto promote the penetration of the oxygen-saturated fluid into the fibreif a very high pressure is maintained, and to subject the pulp to arepeated mixing at this high pressure.

The principles that are applied in the invention are the establishmentin a first high pressure section of a pressure that is greater than the12-15 bar that has been set in most commercial systems as the maximumsuitable operating pressure, and the initiation at this higher pressureof remixing effects in the pulp suspension, with or without the extraaddition of chemicals, with the aim of ensuring that the part of theoxygen that has been dissolved in the fluid phase is held as high aspossible throughout the complete volume of the fluid and that the fibreis stirred in this volume of fluid that has been saturated with oxygensuch that the fluid that has been saturated with oxygen can be allowedto penetrate the fibre in a more effective manner.

It is typical that a pressure greater than 15-20 bar or higher isestablished in this high pressure section.

Other advantages and aims are made clear by the following description ofembodiments.

DESCRIPTION OF DRAWINGS

FIG. 1 shows schematically a system for oxygen delignification in whichthe method according to the invention can be applied;

FIG. 2 shows in principle how oxygen is mixed in to be mixed with thecellulose pulp as finely divided or as a dissolved fraction during theprocess as described by FIG. 1;

FIG. 3 shows how oxygen is consumed in an alternative system with threeremixer positions during the high pressure phase and with maintainedpressure, and with a mixing effect in the final reactor of the highpressure section;

FIG. 4 shows how the oxygen is consumed in a further alternative systemwith only 14 kg of batchwise added oxygen and two remixing positionsplaced close to each other;

FIG. 5 shows how oxygen is consumed in a two-reactor system with a highpressure zone and a low pressure-zone without repeated remixing in thehigh pressure zone.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an oxygen delignification system between a preceding washW₁ and a subsequent wash W₂/W₃, with a number of reactors R₁, R₂, R₃ andR₄ between the washes.

The pulp is passed after the first wash W₁ to an atmospheric storagetower ST (or to an atmospheric pulp chute).

It is appropriate that the alkali that must be added batchwise to theoxygen is added batchwise at the bottom of the storage tower(NaOH_(MAIN)) after which all subsequent pumps (P1/P2) contribute toefficient mixing, since pumps are good mixers of fluid additions suchas, for example, alkali (NaOH).

The three first reactors R₁, R₂, and R₃ are part of a high pressuresection that establishes its pressure by two pumps P1 and P2 connectedin series. The first pump P1 is a fluidising MC-pump with degassing,which not only is necessary in order to fluidise the MC-pulp such thatit can be pumped, but also is used to separate out air (Air) from thepulp, which air otherwise would influence the process negatively fromthe point of view of delignification (not a high level of oxygen or acontent of other residual gases) and reduce the opportunity ofpressurising the pulp in an optimal manner. Also the second pump may bea fluidising MC-pump, but in this case without degassing, although it isappropriate that it is a conventional centrifugal pump optimised forpressurisation, since the pulp has already been fluidised and degassedby the first pump. It is appropriate that the second pump has asignificantly higher pumping efficiency, normally a pumping efficiencythat lies 10-20% higher than that of the first pump.

The pumps P1, P2 establish initially in the high pressure section R₁-R₃a pressure that lies well over 15.0 bar, typically in the interval 17-30bar and. preferably in the interval 17-25 bar. The pressure may behigher, but it is limited by practical considerations to 17-20 bar withtwo pumps connected in series. If a pressure level of, for example, 18bar is to be established after the pumps P1/P2, a suitable dimensioningand choice of pumps can be carried out such that the first fluidisingMC-pump P1 establishes a pressure height of 6-8 bar (approximately30-40%), and the subsequent centrifugal pump establishes a pressureheight of 12-14 bar (approximately 60-70%), i.e. approximately one thirdof the total required pressure height is achieved in the first MC-pumpand the remaining pressure height in the second pump. If a higherpressure is required, several pumps can be connected in series, wherealso a third and final pump can be a centrifugal pump.

It is preferable that oxygen is added after the pressurisation by thepumps P1/P2 to the pressurised cellulose pulp through a principal mixerM1. It is appropriate that the mixer M1 should be a high intensity mixerwith retention times of at least 0.1-2.0 seconds, which mixes oxygenevenly into the cellulose pulp in a powerful shear force field.

The pressurised pulp is led from the first principal mixer M1 at apressure of approximately 18 bar with the oxygen that has been mixedinto it to a first reactor R₁ in which the pulp flows upwards with afirst retention time t₁ in the reactor R₁.

After the pulp has passed through the first reactor R₁ the pulp is ledto a second remixing location in the form of the mixer M2. This mixermay be of a simpler type than the principal mixer M1, and the principalaim is to obtain a remixing effect in order to increase the dissolvedfraction of oxygen and to promote the penetration of the saturated fluidphase into the cellulose fibre. The fall in pressure across this mixeris to be kept as low as possible, preferably well under 1 bar, and themixer may in its simplest form be a static mixer, possibly in the formof a half-closed valve. The mixer M2 may also be a small and simple pump(with a limited build-up of pressure that corresponds to the fall inpressure in the pipes and system up to this point), or it may be anagitation and fluidising mixer.

The pressurised pulp is led from the second remixing location by themixer M2 at a pressure of approximately 17 bar with its remixed oxygento a second reactor R₂ in which the pulp flows upwards with a secondretention time t₂ in the reactor R₂.

After the pulp has passed the second reactor R₂ the pulp is led to athird remixing position in the form of the mixer M3. This mixer may beof the same type as the mixer M2 with the same aim and the same fall inpressure. The pressurised pulp is led from the third remixing locationat a pressure of approximately 16 bar with its remixed oxygen to a thirdreactor R₃ in which the pulp flows upwards with a third retention timet₃ in the reactor R₃.

The high pressure section ends after the third reactor R₃ and acontrolled and managed reduction in pressure is thereafter carried outbefore the concluding low pressure section in at least one final reactorR₄.

The reduction in pressure can be implemented through at least one valve,although it is preferable that several valves V₁, V₂ and V₃ are used,where it is ensured that the fall in pressure is kept as low as possibleacross each valve in order to avoid large and sudden falls in pressurethat risk unnecessarily the flashing out of gas in the form of largebubbles. Several valves in series ensures that each valve also providesa stirring mixing effect in the turbulence that is formed, whichpartially counteracts the negative effects of the fall in pressure bymaintaining the oxygen evenly distributed in the form of small bubbles.

The low pressure section is, however, to continue to have a relativelyhigh pressure in order to be able to maintain a finely distributedresidual amount of the oxygen in the cellulose pulp, but it is to have asufficiently low pressure such that the pulp can be heated by theaddition of direct steam (MP steam) from the medium pressure steamnetwork of the mill. The medium pressure steam in these networks isnormally held at a pressure of 10-12 bar, and a positive pressuredifference of at least 1-2 bar is required in order to be able tointroduce steam into the pulp. The pressure difference required dependson the particular mixer M4, where this medium pressure steam is added tothe pulp after the establishment of the correct pressure following thecontrolled pressure reduction after the valves V1-V3.

It is appropriate that a pressure less than 10-12 bar is establishedafter the high pressure section, the level being determined by thepressure in the steam network for medium pressure steam at the mill, andthis pressure after the high pressure section should be at least 1-2 barlower than the pressure in this steam network, with conventional mixingmethods for the steam. If the pressure difference is lower, anarrangement must be used for the addition of steam in which steam isadded into the pulp through sluices without the risk of the pulp flowingout against the flow of steam.

The addition of steam entails the raising of the temperature of thecellulose pulp by at least 5° C. followed by the leading of the heatedpulp to a reactor system in a low pressure part, with a retention timet₄ that exceeds the total retention time (t₁+t₂+t₃) in the high pressuresection.

The reactors R₁, R₂ and R₃ in the high pressure section are sodimensioned that the cellulose pulp is given successively longerretention times for the cellulose pulp, such that when the number ofhigh pressure reactors is X, then the retention time for the reactorst_(1−X) is such that t₁<t₂< . . . t_(X), where t₁ is the retention timein reactor R₁, etc. This ensures adaptation to the reaction process, inthe form of consumption of the added chemicals, something that occursvery rapidly initially and subsequently falls essentially exponentially.Suitable retention times in the reactors R₁ to R_(X) in the highpressure section can be approximately expressed as:

-   -   t_(min)=1 minute for t₁, after which (t_(x)=2*t_(x−1)) and        t_(max)=X*10 minutes;        -   (t₁=1-10, t₂=2-20; t₃=4-30; t₄4=8-40 min., etc),    -   where t_(X)<t_(x+1).

With the aim of further improving the fraction of dissolved oxygen inthe complete fluid volume in the high pressure section, a stirrer can beplaced in at least one high pressure reactor that acts in the main partof the reactor volume (it is to act in greater than 50% of the reactorvolume), either in the form of a mechanical stirrer (S) or ahydrodynamic stirrer that at least circulates free fluid in the reactor.This stirrer may be present, for example, in the largest reactor in thehigh pressure section, or in the reaction in which the consumption ismost rapid (which is the first reactor), or in all reactors. Themechanical stirrer can be realised in the form of a rotating shaft witharms that extend radially in the reactor, which stirrer is driven at arather moderate rate of revolution of 10-100 rpm and does not exert anyfluidising effect on the cellulose pulp.

The retention time in the heated low pressure section is greater thanthe total time in the high pressure section. Given a total retentiontime of at least 3-10 minutes in the high pressure section, a suitableminimum retention time in the low pressure section is at least 30minutes, which gives a total retention time of at least 40 minutes. Thehigh pressure section for these minimum conditions can consist of afirst reactor with a retention time of at least one minute, remixing,followed by a second reactor with a retention time of at least 2 minutesand, after a further remixing, a final third reactor in the highpressure section with a retention time of at least 4 minutes. The lowpressure section may have a retention time in the interval 30-120minutes, preferably 60-90 minutes.

In a plant that delignifies 3,000 tones of MC-pulp at a consistency of10% every day, the flow through the process is just over 18 cubic metresper minute. Pipes with a diameter of approximately 1.2 metres arenormally used for such a level of production, and these containapproximately 1.1 cubic metres per metre of pipe. A first reactor whichhas then be implemented in the form of a single pipe with a retentiontime of 1 minute will then be approximately 16 m long. It is oftenattempted to realise the reactors as pipes, which is the principalreason for keeping the retention times in the reactors low. A pipe oflength 16 metres can be formed in the shape of a U-bend in a verticalplane, or it can be located in a horizontal plane.

The pressure in the cellulose pulp is released after the low pressuresection at the outlet from the reactor R4 and the pulp is fed to anatmospheric pulp chute (or to a storage tower) that can lead offresidual gases, after which the pulp from which the pressure has beenreleased is pumped by pump P3 to the subsequent washing system, which isshown in FIG. 1 in the form of a pressure diffuser W₂ and a washingpress W₃ placed in series. This combination can become relevant if veryhigh requirements are placed on the cleanliness of the pulp, forexample, if the pulp is to be used as packaging for foodstuffs. It is,however, often sufficient with a single washing machine, and thisreduces the investment costs. Washing filtrate F3 is led in aconventional manner from the final wash W₃ as washing fluid F2 for thenext to last wash W₂, i.e. the washing filtrate is led as acountercurrent flow relative to the flow of pulp. The washing filtrateF1 from the wash W₂ directly after the final reactor R₄ is led in anequivalent manner to a wash W₁ before the oxygen delignification as atleast one of washing fluid and dilution fluid for the storage tower STafter this wash W₁. The pulp that has been subject to oxygendelignification and washed is then pumped by the pump P5 to subsequentbleaching stages in the bleaching sequence.

FIG. 2 shows in principle how oxygen is consumed in a system asportrayed in FIG. 1, with the amount of oxygen plotted along the Y-axisand time plotted along the X-axis. The total batch of oxygen that isadded in the mixer M1 is here 20 kg per tonne of pulp, which batch sizelies in the upper region of the interval of 14-20 kg per tonne of pulpthat is conventionally added batchwise to a oxygen delignificationstage. It is not normally possible to add much more than 30-35 kg oxygenper tonne of pulp to a mixing-in position at a pressure of 12 bar sincesuch large quantities of gas readily give rise to the formation ofchannels in subsequent reactors, something that establishes a centralflow through the reactor with very high speed, and a retention time inthe reactor of only a fraction of the intended retention time. The highpressure that is applied according to the invention during the remixingand in the subsequent reactor results also in the ability to add greateramounts of gas without the risk of channels forming, or with a heavilyreduced risk of channels forming, compared with systems having a lowerpressure, given the same batchwise addition of oxygen. The residualquantity of oxygen that has not been consumed at any moment largelyfollows the line O₂ RES_(TOT) and the consumption is initially veryrapid. The symbols I, II and III denote the retention times in thereactors R₁, R₂ and R₃, respectively.

The maximum amount that can be dissolved in the fluid phase at theprevalent pressure is shown in FIG. 2 by the grey-shaded region O2Liq_(MAX). At a pressure of 20 bar, at 80° C. and at pH 10,approximately 0.5 kg of oxygen per cubic metre of fluid can be dissolvedin the fluid phase. (The amount at a pressure of 10 bar is 0.25 kg).There is 9 cubic metres of fluid per tonne of pulp at a pulp consistencyof 10%, and this means that a maximum of 4.5 kg of oxygen per tonne ofpulp can be dissolved at a pressure or 20 bar. The amount 4.5 kg of atotal batch size of 20 kg is equivalent to as much as 22.5% of the totalbatch size. The solubility of oxygen in the fluid phase is relativelylow compared with those of many other bleaching agents, such as chlorinedioxide and other fluid-phase bleaching agents. The process in whichoxygen passes from the gas phase to the fluid phase is relatively slow,but it can be accelerated through vigorous agitation in fluidisingmixers. For this reason, thus, the first mixer M1 should either be ahigh-fluidising mixer with a relatively short retention time of 0.1-2.0seconds, which vigorously agitates the mixture of pulp and gas in anintensive field of shear forces applied in a thin flow slit, or itshould be a more moderate continuous agitation that occurs for a longerperiod of 1-10 seconds.

Given an optimal mixing in the mixer M1, it is possible to assume thatthe amount of oxygen that has been dissolved in the fluid phase liesclose to the amount that theoretically can be dissolved in the fluidphase. The consumption of that amount of oxygen that has been dissolvedin the fluid phase, however, takes place very rapidly and considerablymore rapidly than the transition of the oxygen from the gas phase to thefluid phase, and the fraction of remaining dissolved oxygen in the fluidphase follows the curve O₂ Liq RES for this reason. The oxygen that isto react with the fibre wall in the cellulose is, therefore, depletedfar too rapidly, and this results in the oxygen delignification of thecellulose fibre occurring under conditions that are by no meansadvantageous. A remixing in the mixer M2 is for this reason activated,such that it is possible to increase the fraction of oxygen that isdissolved in the fluid phase. Also this dissolved quantity is consumedduring the period II in the reactor R₂ and a further remixing with themixer M3 for this reason takes place, in order again to increase thefraction of oxygen that is dissolved in the fluid phase. The oxygen thathas been dissolved by the mixer M3 is subsequently consumed during theperiod III in reactor R₃.

The system in FIG. 1 is of conventional type, and the pressure thusfalls through the system due to pressure loss in pipes, etc. Thepressure falls for this reason in FIG. 1 from reactor R₁ to R₂ byapproximately 1 bar, and it falls from R₂ to R₃ with approximately 1bar, exclusive of any pressure fall that depends on loss of staticheight. There will also be a pressure fall of 1 bar in a reactor with aheight of 10 metres due to the difference in the static heights of theinlet and the outlet of the reactor. A high starting pressure of 17 barthus gives approximately 16 bar in reactor R₂ and 15 bar in reactor R₃.The fall in pressure through the high pressure section (HP) of thesystem (corresponding to the reactors R₁, R₂, R₃) is to be minimisedsince the principle of the invention is to maintain the pressure at ahigh level as far as possible throughout the complete high pressuresection HP, and to activate the remixing operations at a maintainedpressure in order to maintain the fraction of oxygen that has beendissolved in the fluid phase as high as possible as far as it ispractically possible.

The pressure in itself is important in order to facilitate thetransition from gas phase to fluid phase since it is possible tomaintain the area of contact between the gas phase and the fluid phaseat a high level if undissolved oxygen (distributed in the suspension aseither visible or invisible foam or bubbles) can be maintained in theform of small bubbles and counteract the aggregation of these into largeaggregates of gas, which happens if, among other effects, the pressureis reduced.

If the gas can be maintained in the form of bubbles with a diameter of0.1 mm instead of 1 mm, the area of contact between the gas phase andthe fluid phase can be increased by a factor of 100. It is also possibleto maintain small bubbles distributed through the complete volume of thesuspension, and they can penetrate fibre walls more easily.

The pressure is reduced after the high pressure section HP to a lowerlevel in the low pressure section LP, to a pressure at which mediumpressure steam can be used to heat the pulp suspension directly. At anappropriate pressure of 10 bar in the low pressure section, only half ofthe quantity of oxygen that can be dissolved at 20 bar can be held insolution. The remaining quantity of oxygen at this process position,however, is relatively low, due to the high consumption in the highpressure section. The amount of oxygen that theoretically can bedissolved at a pressure of 10 bar, however, does amount to 40-60% of theresidual amount of oxygen that remains, and for this reason it may bethe case in certain systems that it is desirable to add a small amountof further oxygen before the low pressure section.

In summary, FIG. 2 shows how the fraction of oxygen dissolved in thefluid phase can be maintained at a high level by repeated remixing notonly by remixing but also by retaining a high pressure. Both of theseconditions are necessary in order to be able to dissolve a fraction ashigh as just over 20% of the total addition of oxygen and maintainingthe amount of oxygen in the fluid phase at a high level throughout thecomplete high pressure section. Advantageous conditions fordelignification of the fibre wall at its contact with the fluid phase,which also penetrates into and between the fibres, are in this waycreated.

FIG. 3 shows another variant in which three remixing positions betweenfour reactors are used, and a stirrer S is also present in the finalreactor, as is shown schematically in FIG. 1. The same high pressurethroughout the complete high pressure section HP is established in thiscase, and this may be ensured where required by mixers at the remixingpositions that raise the pressure. The mixers DUALOMIX™ from KvaernerPulping AB are mixers of a type that can give a build-up of pressure incertain applications (determined by the size and the flow) and withcertain designs of the mixer. It is possible as an alternative to insertauxiliary pumps between the reactors. In this case, again, 20 kg ofoxygen has been added as a batch at the beginning, but the remixingoperations take place more frequently with shorter and successivelyincreasing retention times between the remixing positions.

FIG. 4 shows a variant with an initial batchwise addition of oxygen of14 kg, maintained pressure throughout the system and more frequentremixing operations than in the variant shown in FIG. 2. Otherwise thisvariant is the same as that shown in FIG. 2.

FIG. 5 shows how the oxygen would be consumed if the method according tothe invention were to be not applied in a two-reactor system with afirst high pressure section and a second low pressure section, i.e. ifremixing does not take place in the high pressure section. The drawingmakes it clear that the amount of oxygen dissolved in the fluid phasefalls rapidly to a very low amount, and delignification of the fibre andthe fibre wall takes place for this reason in conditions that are farfrom ideal in the latter part of the high pressure section.

The main part of the chemicals, both oxygen and alkali, required is, inprinciple, to be added before the first reactor, which quantities ofchemicals are such that alkali is added to obtain an initial pH thatlies well over 9.0 and oxygen is added at an amount of between 5 and 50kg per tonne of pulp. The particular amount added depends on the initialkappa value. When using pulp with an initial kappa value of 40-50 unitsand with a reduction to a kappa value of 8-10, the addition of oxygencan amount to 30-50 kg per tonne of pulp, i.e. an effect on the kappavalue of 1-2 kg per Δkappa and tonne of pulp. If the initial kappa valueis lower, the kappa factor typically lies between 1.5 and 3.0 kg perΔkappa and tonne of pulp. Thus the invention is applied in an oxygendelignification with a kappa factor addition of oxygen that lies in theregion 1.0-3.0 (kg oxygen per Δkappa and tonne of pulp). Alkali must beadded such that the pH is maintained at a final value of 10-10.5 suchthat the alkalinity is maintained at a sufficiently high level duringthe complete process. This normally means that 80-100% of the totalamount of oxygen is added at the first mixing-in position, while 70-90%of the total amount of alkali is added at the same mixing-in position.In certain cases, in particular primarily in cases in which the totalretention time is short, the complete amount of alkali can be added atthis position.

Normally, only alkali is added before the low pressure sectionprincipally for long total retention times, together with a small amountof oxygen if the retention time in the low pressure section is long. Ithas proved to be the case in practice that the vast majority of systemsdisplay improved delignification and an improved strength of the pulp,i.e. an improved selectivity, if a small amount (typically 10%-40% ofthe total amount) is added as a batch at the second stage.

Adaptation of the strategy for additions takes place depending on anumber of factors, such as:

-   -   the initial kappa value, where a higher initial kappa value may        entail a greater number of addition points for alkali or oxygen;    -   the total kappa reduction during the oxygen delignification;    -   the current cellulose being used (deciduous wood, conifer wood,        heavy eucalyptus, etc., where short-fibred deciduous wood may        require more frequent or a greater number of remixing positions        in the high pressure section);    -   the subsequent bleaching stages (the ECF or the TCF sequence,        the use of other alkali bleaching stages of P-stage type, and        the power and the number of alkali extraction stages of        E-/EO-/EOP-type);    -   the properties desired for the bleached pulp;    -   requirements on the COD level of emissions (this may require        more severe conditions in the oxygen treatment despite reduced        selectivity, which may reduce the amount of chlorine dioxide        required in D-stages).

With the aim of obtaining an optimal effect on the cellulose pulp, andof not consuming the chemicals on unnecessary organic material in thefluid phase, the cellulose pulp may be dewatered to give a higherconsistency before the oxygen delignification and it may be rediluted toa medium consistency before the oxygen delignificafion using purefiltrate (filtrate that is obtained from washing stages after the oxygendelignification or other clean process water), which has preferably beenoxidised before this in an oxidising reactor Ox.R. The alkali that isadded may, for the same reason, be added in portions or totally in theform of oxidised white liquor.

The invention may be varied in a number of ways within the framework ofthe invention. For example, types of reactor other than a tower ofupward flow may be used, such as a tower of downward flow, or simplepipes that have been laid in a horizontal plane or in a U-bend in avertical plane.

Batchwise addition of oxygen may be required at all mixing positions inthe system for certain pulps, such as deciduous wood pulps, that aredifficult to delignify. The use of more than three reactors withpreceding mixing operations for each in the high pressure section mayalso be required, as may the use of more than one reactor in thesubsequent low pressure section. However, at least one remixing positionis to be established in the high pressure section with a predeterminedminimum time delay following a preceding principal mixing position,together with a subsequent low pressure section having at least onereactor.

The predetermined time delay between the mixing positions may be adaptedto the current consumption of oxygen dissolved in the fluid phase or tothe total amount of added oxygen, where it is appropriate that aremixing operation can take place as soon as possible after more than30% of the oxygen that has been previously added has been consumed. Thepreferred retention times for the pulp in the reactors R1-R3 that havebeen specified are, however, useful guidelines for the vast majority ofprocesses, which guidelines have been adapted to the rapid consumptionat the beginning of the process, a rate of consumption that graduallydeclines.

The invention may also be applied in a delignification system with ashort initial low pressure section in which a small amount of oxygen,typically considerably less than 40% and preferably less than 20% of thetotal that is added to the oxygen stage, is added before or at the lowpressure section with the aim of oxidising the material that is presentdissolved in the fluid phase. It is possible in this manner to avoidunnecessary consumption of the oxygen that is added at the subsequenthigh pressure section by oxidisable material in the fluid phase, andensure a greater fraction being used for delignification of thecellulose in the high pressure section.

It is preferable that also oxidised white liquor is used as addition ofalkali.

Oxygen delignification in a mill environment can be more readily drivento lower kappa values with the method according to the invention, anddeciduous wood, for example, which in laboratory trials can bedelignified-down to a kappa value of 9, or lower, can approach thispotential reduction in kappa also in a mill environment. The reductionin kappa value in a mill environment can, in certain conditions, beimproved by up to 3 kappa units while retaining the strength of thepulp. It is alternatively possible to obtain the same reduction in kappavalue with a considerably improved pulp strength, or intermediatevariants between these extreme alternatives may be achieved in whichboth the strength of the pulp and the reduction in kappa value areimproved. The costs for the operation of the bleaching line, inparticular the costs for further bleaching agents in subsequentbleaching stages, are considerably reduced if it is possible to reducethe kappa value by a further 2-3 units in the oxygen delignification atan early stage of the bleaching line.

1. A method for continuous alkali oxygen delignification of digestedcellulose pulp and of cellulose pulp that has been washed afterdigestion, comprising: storing pulp in a storage tower or pulp chute atessentially atmospheric pressure, maintaining a medium consistency ofthe pulp in a range of 8-18%, maintaining the cellulose pulp to bedelignified at a kappa value of at least 15 units, the oxygendelignification taking place in a reactor system with several oxygenreactors with a predetermined retention time of the cellulose pulp inthe reactor system, adding alkali to the cellulose pulp in order toobtain an initial pH exceeding 9.0 and adding oxygen to the cellulosepulp at an amount of 5-50 kg per tone of pulp at a position before afirst oxygen reactor in the reactor system, providing the pulp with apredetermined total retention time of greater than 45 minutes in thereactor system, in association with an addition of chemicals and aninitial mixing-in operation, placing the cellulose pulp under pressureat an initial pressure of greater than 15.0 bar, subjecting the pulp tomore than one remixing position where a final pressure after a finalremixing position is at least 13 bar, subjecting the pulp to a minimumretention time in a high pressure section of at least 3-10 minutes,reducing the pressure of the pulp to a pressure that lies under 10-12bar, heating the pulp with steam such that a temperature of the pulp israised by at least 5° C. by the addition of steam, and leading theheated pulp to a reactor system in a low pressure section with aretention time that exceeds the retention time in the high pressuresection.
 2. The method according to claim 1, wherein oxygen, is added tothe cellulose pulp immediately after the initial pressure of more than15 bar has been established.
 3. The method according to claim 2, whereinthe remixing positions are constituted by fluidising mixers, either in aform of a fluidising pump, a fluidising restrictions a fluidising mixeror a restriction in a flow that results in a fall in pressure of lessthan 1 bar.
 4. The method according to claim 3, wherein a first highpressure reactor is located after the initial mixing-in operation, inwhich reactor the cellulose pulp is given a first retention time of t₁,and in that a high pressure reactor follows after the remixing positionsin the high pressure section after each one of the remixing positions.5. The method according to claim 4, wherein the reactors in the highpressure section are dimensioned such that the cellulose pulp is givensuccessively longer retention times, such that when the number ofreactors is X, the retention times are t₁-t_(x) for each relevantreactor R₁-R_(X), where t₁<t₂< . . . t_(x).
 6. The method according toclaim 5, wherein the retention times t₁-t_(x) in the reactors R₁-R_(x)in the high pressure section are expressed as t_(min)=1 minute for t₁,after which (t_(x)=2*t_(x−1)) and T_(max)=X*10 minutes; (t₁=1-10 min.,t₂=2-20 min.; t₃=4-30 min.; t₄=8-40 min.), where t_(x)<t_(x+1).
 7. Themethod according to claim 1 wherein a stirrer is present in at least onehigh pressure reactor, which stirrer acts in a principal part of areactor volume, either in a form of a mechanical stirrer (S) orhydrodynamic stirrers that at least circulate free fluid in the reactor.8. The method according to claim 1 wherein at least one of the oxygenand alkali additions are added to the cellulose pulp in association withthe remixing positions in the high pressure section at an amount that islower than the amount that is added at the initial mixing-in operation,and at least one of the oxygen and alkali additions are added batch-wiseat a beginning of the low pressure section.
 9. The method according toclaim 1 wherein the cellulose pulp is dewatered before the oxygendelignification to a higher consistency and the cellulose pulp isre-diluted before the oxygen delignification to a medium consistencywith pure filtrate that has been previously oxidized, and in that alkaliin a form of oxidized white liquor is used in the oxygendelignification.